refactor: complete Svelte 5 migration to eliminate all deprecation warnings

Migrated all components and routes to Svelte 5 syntax standards:

Event Handlers:
- Updated all deprecated on:* directives to new on* attribute syntax
- Changed on:click → onclick, on:keydown → onkeydown, on:input → oninput
- Updated on:blur, on:focus, on:load, on:submit, on:cancel handlers

Reactive State:
- Added $state() declarations for all reactive variables
- Fixed non-reactive update warnings in layout and component files

Component API:
- Replaced <slot /> with {@render children()} pattern
- Added children prop to components using slots

Accessibility:
- Added id attributes to inputs and for attributes to labels
- Fixed label-control associations across forms
- Removed event listeners from non-interactive elements

HTML Fixes:
- Fixed self-closing textarea tags
- Corrected implicitly closed elements
- Proper element nesting

CSS Cleanup:
- Removed 20+ unused CSS selectors across components
- Cleaned up orphaned styles from refactoring

All vite-plugin-svelte warnings resolved. Codebase now fully compliant with Svelte 5.

src/lib/components/SeasonSelect.svelte

@@ -92,7 +92,7 @@ input[type=checkbox]::after
 {#each months as month}
 	<div class=checkbox_container>
 		<!-- svelte-ignore a11y-no-noninteractive-tabindex-->
-		<label tabindex="0" on:keydown={(event) => do_on_key(event, 'Enter', false, () => {toggle_checkbox_on_key(event)}) } ><input tabindex=-1 type="checkbox" name="checkbox" value="value" on:click={set_season}>{month}</label>
+		<label tabindex="0" onkeydown={(event) => do_on_key(event, 'Enter', false, () => {toggle_checkbox_on_key(event)}) } ><input tabindex=-1 type="checkbox" name="checkbox" value="value" onclick={set_season}>{month}</label>
 	</div>
 {/each}
 </div>